1. Field of the Invention
The present invention relates generally to electronic lamp ballasts, and relates more particularly to an electronic lamp ballast and control for a high intensity discharge lamp.
2. Description of Related Art
High intensity discharge (HID) lamps have been in use for a number of years in a variety of lighting applications. Four types of HID lamps are typically used in practice, including mercury vapor, low pressure sodium, high pressure sodium and metal halide. HID lamps enjoy a number of advantages over incandescent lighting and fluorescent lighting, but also have some comparative limitations. For example, HID lamps typically do not start immediately, but require some warm up time to come to full illumination. In addition, HID lamps typically emit light in several different colors, for example, mercury vapor lamps tend to have a bluish color while sodium lamps tend to have a yellow color output. Metal halide lamps typically have a bright white output and is preferred for a number of applications where light quality is an important factor.
Fluorescent lamps tend to be much larger than HID lamps and have nearly instant start up and restart times. In addition, fluorescent lamp performance tends to be very temperature sensitive, while HID lamps can operate in a variety of environments without significant decreases in performance.
In the past, HID lamps have dominated the lighting market because of the high lumen output, high efficiency and superior light quality provided by metal halide HID lamps. The typical metal halide HID lamp was formerly driven by magnetic ballast, which were not usually optimized for any particular application. Presently, electronic ballasts for metal halide HID lighting applications have been developed that increase the performance and efficiency of the HID lamp, which has the advantageous result of decreasing maintenance and energy costs. The use of electronic ballasts also permits HID lamps to be operated in a dimming mode. Electronic ballast may also be made to adapt to the changing conditions of the lamp over its life, and thus compensate for a typical reduction in light output from the lamp over its lifetime. By compensating for the reduction in light output, the electronic ballast can maintain a higher and consistent light level for the duration of the life of the lamp. Accordingly, electronic ballasts reduce the need for frequent lamp replacement, while improving efficiency, thereby reducing the overall count of lighting fixtures needed to realize a particular application.
In a typical HID lamp, an igniter is used to start the lamp, which improves the lamp life and simplifies operation of the lamp. In prior HID lamps, a large open circuit voltage was needed to ignite the lamp, causing a high lamp current crest factor that contributed to diminishing the lifetime of the lamp. By providing an igniter, the electronic ballast for the HID lamp can operate at a potentially lower voltage that provides a number of advantages. For example, it is typically preferable to operate an electronic ballast for an HID lamp at a given frequency to avoid interference with other electronic equipment and improve the efficiency of the lighting application. Electronic ballasts are also able to maintain lamp power at a particular rated point for the lamp throughout its lifetime. Maintaining lamp power throughout the lifetime of the lamp is important because the intrinsic voltage of the lamp arc tube changes as the lamp ages, and electronic ballasts should be adaptable to maintain a constant power output.
In general, HID lamps have higher ignition voltages than fluorescent lamps, typically in the range of 3 Kv when measured peak-to-peak. HID lamps generally also have no filaments, which avoids the need to preheat filaments, where fluorescent lamps typically require filament preheat. While electronic ballasts for fluorescent lamps are typically operated at 30 to 50 kHz, HID lamps are operated in a range that avoids these frequencies due to the problem of acoustic resonance that can cause damage and catastrophic failure to the lamp. Thus, HID lamps are often operated in the range of low frequencies, typically a few hundred Hertz. In these low frequency ranges, full bridge switching circuits are used to drive HID lamps with a square wave without resonant output circuits. Realizing a good HID electronic ballast design also includes handling the design challenge of high ignition voltage when attempting to ignite a hot HID lamp. In this circumstance, the ignition voltage can rise to voltages on the order of approximately 25 kV, which can be difficult for an electronic ballast to handle.
Another design criteria that an electronic ballast preferably handles is power factor correction (PFC). A typical power converter connected to a line input should ideally draw current and voltage in phase with each other, so that the power converter load appears as a purely resistant load to the power line input. A high power factor, close to unity, for example, indicates that the load on the power line input approaches the characteristics of a resistive load. A unity power factor is desirable to avoid capacitive or inductive impedances that can undermine the quality of the power line input supplied to other devices connected to the power line. Accordingly, an HID ballast should provide power factor correction to avoid excessive impedances on the line input.
It is often the case that an electronic ballast designed to meet the above described design challenges is realized with a number of components and integrated circuits to control the electronic ballast to provide proper operation. For example, each side of a full bridge used to operate an HID lamp typically has its own driver integrated circuit, IC, while another IC is used as a control for power factor correction. Still other ICs are often used to realize a system control for overall operation of the electronic ballast, including driving switches and obtaining feedback. It would be desirable to obtain an electronic ballast for an HID lamp with a simplified construction and reduced component count to improve ballast efficiency and reduce costs.